1. Field of the Invention
The present invention relates to a vehicular suspension control method and apparatus used in a vehicle, such as an automobile, and adapted to detect the condition of a road surface in front of the vehicle and properly control the operation of a suspension.
2. Description of the Related Art
An optical forward road surface sensor described in Published Unexamined Japanese Patent Application No. 60-142208 (U.S. Pat. No. 4,781,465; G.B. Pat. No. 2,151,872; DE. Pat. No. 3,447,015; FR. Pat. No. 2,557,288) and an ultrasonic forward road surface sensor described in Published Unexamined Japanese Patent Application No. 62-131813 are conventionally known as apparatuses for detecting irregularities of a road surface in front of a vehicle. In these prior art examples, it is proposed that the operation of a suspension be controlled depending on the conditions of the road surface in front of the vehicle detected by the forward road surface sensor.
In the former patent application, there is no description of a specific example of the suspension operation control. Disclosed in the latter patent application, on the other hand, is a technique for controlling the damper rigidity of the suspension, more specifically, a vibration damping force which is produced by a shock absorber, when a wheel of the vehicle reaches an irregularity which is detected on the road surface in front of the vehicle, referring to the vehicle speed. According to this technique, the damper rigidity of the suspension is lowered when the irregularity of the road surface in front of the vehicle is detected by the forward road surface sensor. Thus, impulsive vibrations produced in the vehicle body is effectively damped when the wheel gets over irregularities of the road surface, e.g., protuberances or joints in a paved road.
In the latter prior art example described above, the forward road surface sensor is mounted on the front portion of the vehicle at a predetermined angle of incidence, and is used to detect the irregularity of the road surface located in front of the vehicle at a predetermined distance therefrom when the vehicle body is in a predetermined attitude, e.g., when the vehicle is at a stop. The aforementioned control is effected in accordance with this predetermined distance. Since the forward road surface sensor is low-priced, the cost of the whole apparatus can be reduced with ease. When the vehicle actually runs, however, its body undergoes a change of attitude attributable to pitching or nose dive, so that the actual distance between the vehicle and the irregularity of the road surface detected by the forward road surface sensor frequently changes. Thus, a difference is caused between this actual distance and the predetermined distance. In this prior art example, therefore, the damper rigidity of the suspension is not controlled with correct timing when the wheel gets over the irregularity of the road surface.
According to the former prior art example, moreover, a sensing device is used to measure distances from the vehicle to three positions in front of the vehicle, thereby detecting the distances between the vehicle and irregularities of the road surface, as well as the irregularities themselves. Although the use of this device solves the aforesaid problem, it makes the whole apparatus more complicated in construction, more expensive, and therefore, hardly practical.